POTS, but now we have swapped everything over to VOIP, and let me tell you, it's been a learning experience!
With POTS, the local phone company brings in a separate pair of wires for each line, and each one has a different phone number associated with it. Then someone (us!) has to physically connect them to each desk. Keeping track of all those interconnects is just plain ugly.
All I can say, after pulling an all nighter helping to update an old POTS system, I'm glad that Frank convinced us to install VOIP when we moved in here.